The present invention relates to processes for producing polyester resins and in particular poly(ethylene) terephthalate (PET) containing low levels of phosphorus containing additives that is suitable for use in a variety of applications including the manufacture of articles such as containers.
It is well known in the art that polyesters such as poly(ethylene terephthalate) (PET) are useful for many packaging applications. In fact, PET or modified PET is a polymer of choice for making beverage and food containers, particularly carbonated beverage containers.
Poly(ethylene terephthalate) may be derived from multistep processes well known in the art which may include the direct esterification of ethylene glycol and terephthalic acid. In addition, it is known that PET can be modified with additional acidic and/or glycol comonomers, e.g., isophthalic acid (or dimethyl isophthalate), 1,4-cyclohexanedimethanol (CHDM), and the like. Modifying PET with additional comonomers may improve some of the physical properties of the resulting polyesters and provide particularly desired properties in an article formed from the polyester particularly in the areas of crystallization and processability.
Polyesters such as PET are typically formed via three-stage processes. The three preferred stages are often referred to as the esterification stage, the prepolymer stage, and the polycondensation stage.
Each of the stages can employ catalysts and certain additives. For example, since both the prepolymer stage and the polycondensation stage employ the same basic reaction, a variety of polycondensation catalyst system are recognized in connection with the prepolymer and polycondensation stages. Examples of such catalysts include titanium, gallium, germanium, tin, and antimony compounds. The use of additives is also known in the art. Additives that are recognized in the art include phosphorus-containing stabilizers such as phosphates and phosphoric acid. In this regard, such phosphorus-containing additives are considered interchangeable.
As background in this regard, attention is directed towards the following patents that discuss the use of phosphorus-containing additives:
U.S. Pat. No. 4,499,226, issued Feb. 12, 1985, and assigned to Goodyear Tire and Rubber Company, discloses process for producing high clarity colorless polyesters which include the use of polycondensation catalysts, cobalt-containing compounds and phosphorus-containing additives.
German Patent Application 195 37 930 A1, opened to public inspection on Apr. 17, 1997, and assigned to Zimmer, AG, also discloses a continuous process for the production of transparent polyesters in which unpurified diol that has been eliminated during the polycondensation stage can be internally recycled into the process.
U.S. Pat. No. 4,110,316, issued Aug. 29, 1978 and assigned to E.I. Dupont de Nemours and Company, discloses a process for producing fiber-forming polyester from ethylene glycol and terephthalic acid that includes the use of color inhibitors such as phosphoric acid and triphenyl phosphite.
U.S. Pat. No. 5,243,022, issued Sep. 7, 1993 and assigned to Korea Institute of Science and Technology, discloses a method for forming polyesters that involves forming prepolymers from a first portion of esterification product in the presence of certain catalysts and stabilizers. The prepolymers are then polycondensed together with a second portion of esterification product to form the polyester.
U.S. Pat. No. 5,235,027, issued Aug. 10, 1993 and assigned to Zimmer Aktiegesellschaft, discloses a process for making a specific modified copolyethylene terephthalate that includes the addition of a phosphorus-oxygen compound before polycondensation in an amount that corresponds to a Sb:P weight ratio of at least four.
However, careful consideration of the art reveals serious errors, inconsistencies and contradictions in the way the art views phosphorus-containing additives, particularly the use of low levels of acidic additives in polyester formation.
The present invention is based, in part, on the surprising discovery that the choice of phosphorus-containing additive, when employed in connection with certain polymerization catalysts, can have a significant impact on reaction rate of the polymerization process as well as the clarity of the resulting polyester. Further, it was surprisingly found that the optimal addition sequence of the catalyst and phosphorus-containing additive was impacted by the both choice and level of phosphorus-containing additive.
One aspect of the present invention relates to a process that includes:
(a) esterifying at least one dicarboxylic acid component and at least one diol component; and
(b) polymerizing the product of step (a) under conditions effective to provide a polyester resin,
where polymerization step (b) occurs in the presence of (i) an antimony-based polymerization catalyst and (ii) an acidic phosphorus-containing additive, with the catalyst (i) being added prior to, or together with, the additive (ii), and the acidic phosphorus containing additive (ii) which is selected such that the reaction rate of the polymerization step (b) increases with a decreasing amount of additive (ii). The additive is preferably present in an amount not greater than 15 ppm by weight based on elemental phosphorus in the resulting polymer.
Another aspect of the present invention involves a process for making a polyester resin including the steps of:
(a) esterifying at least one dicarboxylic acid component and at least one diol component; and
(b) polymerizing the product of step (a) under conditions effective to provide a polyester resin,
where the polymerization step (b) occurs in the presence of (i) an antimony-based polymerization catalyst and (ii) an acidic phosphorus-containing additive, with the catalyst (i) being added prior to, or together with, the additive (ii), the amount of additive (ii) being present in an amount such that the polyester resin that is at least substantially free of inorganic compounds which can be formed by the reaction of the catalyst (i) and additive (ii). The amount is preferably not greater than about 15 ppm based on elemental phosphorus in the resulting polyester.
The processes can include a prepolymer stage between steps (a) and (b). Moreover, the process can further include additional steps such as solid-phase polymerization of the polyester resin from step (b).
In other aspects of the present invention, the antimony-based polymerization catalyst (i) is preferably added prior to the acidic phosphorus-containing additive (ii) and the polymerization step (b) is preferably performed in the absence of added cobalt compound(s).
In another aspect, the present invention relates to a polyester, and in particular, a poly(ethylene terephthalate) (PET) resin or a modified PET resin which is preferably made by the inventive process. In this regard, one or more phosphorus compounds are present in an amount not greater than about 25 ppm, more preferably not greater than about 15 ppm based on elemental phosphorus in the resin. In addition, organic toners, e.g., organic red and/or blue toners, are present in a preferred amount of 0.5 of 10 ppm. Finally, the polymer is preferably at least substantially free of antimony phosphate compounds. The polyester resin preferably has an intrinsic viscosity of about 0.4 to 1.2 dL/g measured at 25 C by dissolving 250 mg of polyester in 50 ml or a solvent consisting of a 60:40 ratio by weight of phenol and tetrachloroethane.